The invention relates to the field of mode-locking, and in particular to a self-starting quasi-synchronously pumped Kerr-lens modelocked laser.
Dispersion managed Kerr-lens modelocked (DM-KLM) Titanium-Sapphire (Ti:sapphire) lasers are the work horses in the domain of sub-30 fs laser pulses. Since the first observation of Kerr-lens modelocking (KLM), continuous laser development has led to the generation of octave-spanning spectra and 5 fs pulses directly from the oscillator. DM-KLM lasers exploit the intensity dependent nonlinear refractive index in conjunction with a careful management of the distribution of discrete dispersive elements inside the cavity. In the time domain, this leads to self-phase modulation and hence additional spectral broadening, whereas in the transverse spatial beam dimensions, the build-up of a Kerr-lens together with a suitable resonator geometry enables efficient gain modulation to favor pulsed operation in comparison to continuous wave (CW) operation. However, KLM lasers with pulse durations below a few tens of femtoseconds, i.e. the few-cycle regime, are generally not self-starting and usually require external perturbations to initiate modelocking.
Though KLM allows for a large modulation of the effective gain up to several tens of percent, it is generally not self-starting. A parameter that characterizes the self-starting ability is the so-called modelocking driving force and is defined as (d(Δg)/dI) for I→0, with I the intensity and Δg the gain modulation. In sub-10 fs lasers (less than four optical cycles), the modelocking driving force is designed to be small in order not to overdrive the KLM when the laser transitions from continuous wave operation to pulsed operation.
So far, three different approaches have been presented to overcome the self-starting problem in KLM lasers. One approach is to maximize the modelocking driving force by proper cavity alignment. A particular resonator design enables a maximization of the nonlinear mode variation and consequently dynamic loss modulation, achieving self-starting in a KLM Ti:sapphire when operating close enough to the stability edge. This is only possible down to pulse durations of about 20-40 fs until KLM is overdriven, resulting in a non-continuous modelocking. This means that the nonlinearity in the laser crystal becomes excessive and leads to multiple pulsing and/or modelocked Q-switching.
Alternatively, one can use a semiconductor saturable absorber mirror (SESAM) or saturable Bragg reflector (SBR) inside the cavity. The laser then exhibits self-starting modelocking because SESAMs/SBRs provide large modelocking driving forces in the initial pulse build-up phase. Saturation of the SBR after pulse build-up doesn't harm the laser dynamics, because KLM is taking over the pulse shaping. Another advantage of this method is a relaxed cavity alignment in contrast to purely DM-KLM lasers. A drawback is the bandwidth limitation introduced by these devices that can only be overcome by using non-conventional fabrication procedures which are still under development
A third approach to attain self-starting DM-KLM is (quasi-) synchronous pumping. The term “quasi” accounts for the fact that precise synchronization of the Ti:sapphire laser is not necessary. Self-starting 30 fs pulses with quasi-synchronous pumping have been demonstrated earlier, 30 fs correspond to more than 10 optical cycles at Ti:sapphire wavelength.